The reproduction of continuous tone images is, and has been for a considerable number of years, a major concern in the photographic arts.
A continuous tone image is a positive or negative image, e.g., an opaque print or transparency which is composed of a range of densities from black through gray to white, wherein the grays are formed by forming varying amounts of colorant, e.g., silver compounds, dye or pigment. A continuous tone reproduction contrasts with a line reproduction which is composed of only two tones, black (or a color) and a background color, e.g., white. The same applies to multi-color line images; although there are several colors, each is present only in one depth. The instant invention is directed to a method of half-tone reproduction which simulates a continuous tone image. The term "simulates" is used herein inasmuch as a half-tone image, when viewed at the correct distance, appears to be the result of varying densities. Upon closer examination, however, it becomes obvious that the densities are in fact integrated areas of black and white.
Of the various ways of creating half-tone images, one of the most well known is by screening. A half-tone screen is a line or dot pattern used to convert the continuous tones of varying darkness in a photograph, etc., into a discontinuous pattern of constant density but varying area. In a half-tone image lighter or darker tones are reproduced by smaller or larger dots or lines --which, through being uniformly spaced, occupy a greater or lesser proportion of a given unit area.
Half-tone images can be produced in many ways; the most usual is to convert the continuous tone into a regular dot pattern. In the past, several different structures have been used to produce this pattern, e.g., cross-line screens, gauze, linen and wire. When a cylindrical lenticular lens as proposed herein is used to create a soft line pattern on an imaging member, a "zipper-toned" image is created which is closely related to the well-known soft dot pattern.
The instant invention calls for the use of a lens system for producing the screening effects set forth above. A perfect lens is one which most nearly shows an image of a point as a point and a straight line as a straight line, subject to the faults, or aberrations, inherent in any lens which tend to reproduce a point as a patch, and a straight line as a more or less curved band.
Aberrations which affect an image point on the axis of the lens are classified as axial aberrations. The principal axial aberrations are chromatic and spherical.
Chromatic aberrations merely reflect the fact that a single lens made from a single type of an optical glass will refract blue rays more strongly than green rays, which in turn are refracted more strongly than red rays. Thus, a three-dimensional special positioning of the colored rays results and is referred to as chromatic aberration. Spherical aberration involves the phenomenon that rays coming from an object on the axis of a lens and going through the center of the lens come to a focus at a certain point on the axis of the lens. Rays from an axial object going through the lens near the edges should come to a focus at the same point, but in practice, because of spherical aberration, they tend to come toward a different point of focus. The difference between these focal points is the spherical aberration of the lens. Spherical aberration inceases with the lens aperture. In a simple converging lens, spherical aberration causes the rays farthest from the lens axis to convert more strongly, and to come to a focus nearer the lens than the central rays close to the lens axis. The image is never fully sharp. Spherical aberration does not vary with image size, but with the square of the aperture.
One additional known method used to screen images is the employment of a lenticular lens array. Such an array is one which uses a lenticular screen to break up an image into linear and area components which are subsequently recombined. The purpose of splitting is usually to accommodate two or more images interspersed in each other on the same area. Uses of a lenticular screen or lenticular array are usually for lenticular color photography, stero photography, image disection and multiple image storage. For example, see U.S. Pat. No. 3,413,117, which, in FIG. 7, discloses the use of a lenticular screen in an image deformation system. This patent is specifically concerned with the formation of color images in a thermoplastic deformation system, and requires the creation of relatively perfect images on the surface of the imaging member. The perfection of the image is indicated by the statement therein that " . . . light incident upon each area under each lens-like embossing 36 reacts with the recording layer to provide a stress pattern having a point-to-point correspondence with the image pattern . . . ." Furthermore, it is noted that this patent specifically calls for contact between the lenticular element and the image receiving surface, and that the read-out system is illumination through the film and lens with, or without, Schlieren optics. Also see U.S. Pat. Nos. 1,746,584, 992,151 and 1,749,278.
Generally, the main component of a lenticular system is the lenticular screen itself, consisting of a transparent support embossed with a regular pattern of lens surfaces. Usually these are cylindrical running across the screen in one direction as strips. While several obvious materials are suitable for construction of such a lens, usually they are made of plastic. By embossing a second set of linear elements that run at angles to the first, a lenticular screen consisting of individual lens elements is obtained. Additionally, it should be noted that lenses of this type can be made by any of a number of processes including embossing, extrusion and casting.
When a lenticular screen is placed in front of, and in contact with, the surface on which the camera lens projects an image, the individual lenticular elements break up the image into lines or points. They concentrate the image components into a smaller area, leaving spaces between them. Additional images can be recorded in the spaces by slightly displacing the lens laterally or by moving the object in front of the lens. The record or recorded image then contains a series of interlaced images which can be reconstituted by observation through a similar lenticular screen. The lenticular screen breaks up the image into line elements with spaces in between. If the screen is moved, a new set of line elements is formed in the spaces between the first set.